Connected: An Internet Encyclopedia
RFC 1144 - C More aggressive compression

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### C More aggressive compression

As noted in sec. 3.2.2, easily detected patterns exist in the stream of compressed headers, indicating that more compression could be done. Would this be worthwhile?

The average compressed datagram has only seven bits of header./50/ The framing must be at least one bit (to encode the `type') and will probably be more like two to three bytes. In most interesting cases there will be at least one byte of data. Finally, the end-to-end check---the TCP checksum---must be passed through unmodified./51/

The framing, data and checksum will remain even if the header is completely compressed out so the change in average packet size is, at best, four bytes down to three bytes and one bit --- roughly a 25% improvement in delay./52/ While this may seem significant, on a 2400 bps line it means that typing echo response takes 25 rather than 29 ms. At the present stage of human evolution, this difference is not detectable.

However, the author sheepishly admits to perverting this compression scheme for a very special case data-acquisition problem: We had an instrument and control package floating at 200KV, communicating with ground level via a telemetry system. For many reasons (multiplexed communication, pipelining, error recovery, availability of well tested implementations, etc.), it was convenient to talk to the package using TCP/IP. However, since the primary use of the telemetry link was data acquisition, it was designed with an uplink channel capacity <0.5% the downlink's. To meet application delay budgets, data packets were 100 bytes and, since TCP acks every other packet, the relative uplink bandwidth for acks is a/200 where `a' is the total size of ack packets. Using the scheme in this paper, the smallest ack is four bytes which would imply an uplink bandwidth 2% of the downlink. This wasn't possible so we used the scheme described in footnote 15: If the first bit of the frame was one, it meant `same compressed header as last time'. Otherwise the next two bits gave one of the types described in sec. 3.2. Since the link had excellent forward error correction and traffic made only a single hop, the TCP checksum was compressed out (blush!) of the `same header' packet types/53/ so the total header size for these packets was one bit. Over several months of operation, more than 99% of the 40 byte TCP/IP headers were compressed down to one bit./54/

50. | Tests run with several million packets from a mixed traffic load (i.e., statistics kept on a year's traffic from my home to work) show that 80% of packets use one of the two special encodings and, thus, the only header is the change mask.
51. | If someone tries to sell you a scheme that compresses the TCP checksum `Just say no'. Some poor fool has yet to have the sad experience that reveals the end-to-end argument is gospel truth. Worse, since the fool is subverting your end-to-end error check, you may pay the price for this education and they will be none the wiser. What does it profit a man to gain two byte times of delay and lose peace of mind?
52. | Note again that we must be concerned about interactive delay to be making this argument: Bulk data transfer performance will be dominated by the time to send the data and the difference between three and four byte headers on a datagram containing tens or hundreds of data bytes is, practically, no difference.
53. | The checksum was re-generated in the decompressor and, of course, the `toss' logic was made considerably more aggressive to prevent error propagation.
54. | We have heard the suggestion that `real-time' needs require abandoning TCP/IP in favor of a `light-weight' protocol with smaller headers. It is difficult to envision a protocol that averages less than one header bit per packet.

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Connected: An Internet Encyclopedia
RFC 1144 - C More aggressive compression